Contaminants such as HC (hydrocarbons), CO (carbon monoxide), NOx (nitrogen oxide), and PM (particulate matter) are contained in exhaust gases discharged from diesel engines. Among these contaminants, NOx cannot be purified by three-way catalysts, which have been put to practical use in oxidation catalysts and gasoline automobiles, without difficulties, and, thus, selective reduction-type NOx catalysts (hereinafter referred to as “SCR catalysts”) are under development as promising NOx-purifying catalysts.
Catalysts that have a honeycomb structure and comprise an active ingredient such as V (vanadium), Cr (chromium), Mo (molybdenum), Mn (manganese), Fe (iron), Ni (nickel), Cu (copper), Ag (silver), Au (gold), Pd (palladium), Y (yttrium), Ce (cerium), Nd (neodymium), W (tungsten), In (indium), or Ir (iridium) supported on a carrier, for example, TiO2, two-way composite oxides such as SiO2—TiO2, WO3—TiO2, or SiO2—TiO2, or three-way composite oxides such as WO3—SiO2—TiO2, or MoO3—SiO2—TiO2 are known as SCR catalysts. According to the following formulae, these exhaust gas treatment catalysts reduce NOx in the presence of a reducing agent such as ammonia to convert NOx to nitrogen gas and thus to purify NOx.4NO+4NH3+O2→4N2+6H2O  (1)NO+NO2+2NH3→2N2+3H2O  (2)6NO2+8NH3→7N2+12H2O  (3)
Catalysts comprising a monolith carrier with a supported layer formed thereon, the supported layer being formed of fine particles having catalytic activity such as fine particles of zeolite, are also known.
Further, Japanese Patent Application Laid-Open No. 33664/2003 (Patent Document 1) describes that alumina, zirconia, titania, zeolite, SiC, SiN, mullite, lithium aluminum silicate (LAS), titanium phosphate, perovskite, spinel, chamotte, and non-oriented cordierite are usable as a main constituent material for cell walls of honeycomb catalysts for exhaust gas purification and, among them, titanium oxide, zeolite, and alumina are preferred.
Patent Document 1 describes that, for example, X-type, Y-type, ZSM-5-type, and β-type zeolites are usable as the zeolite, that, however, minimizing the content of the alkali ingredient is important from the viewpoint of heat resistance, that the SiO2/Al2O3 ratio is preferably not less than 25, that AIPO, SAPO, metallosilicates, and layered compounds are also usable, and that catalysts with the above catalytically active ingredients supported thereon by ion exchange are also preferred.
Japanese translation of PCT publication No. 519817/2009 (Patent Document 2) discloses hydrothermally stable metal-treated zeolite catalysts for selective NOx reduction obtained by subjecting zeolite to metal ion exchange at a pH value around 3 and then hydrothermally treating the ion-exchanged zeolite at an elevated temperature of 540° C. or above.
Japanese domestic re-publication of PCT international application No. 011575/2006 (Patent Document 3) discloses denitration catalysts comprising ferric oxide supported on a β-type zeolite carrier subjected to iron ion exchange.
Conventional crystalline porous material catalysts such as the above-described zeolites, when used at 700° C. or above in a reaction that produces water, cause lowered crystallinity and specific surface area that in turn results in lowered activity. Accordingly, the development of catalysts that are hydrothermally stable and can maintain high activity for a long period of time has been demanded.
It is known that crystalline silicoaluminophosphates, even when used as catalyst carriers at elevated temperatures, cause only relatively small lowering in crystallinity and specific surface area and are stable.
Ion exchange, impregnation, and precipitation methods are known as usable in the production of catalysts comprising a metal supported on crystalline silicoaluminophosphates as a carrier, as with crystalline silicoaluminophosphates. The following facts, however, have been found: (1) a method in which ion-exchange is carried out in an aqueous metal salt solution is disadvantageous in that a metal in an amount large enough to provide satisfactory activity cannot be supported, (2) a method in which a crystalline silicoaluminophosphate is dispersed in an aqueous metal salt solution to hydrolyze the metal salt and to precipitate a metal hydroxide is disadvantageous in that the crystallinity of the crystalline silicoaluminophosphate is sacrificed, and (3) a method in which a crystalline silicoaluminophosphate is impregnated with a metal salt is disadvantageous in that satisfactory activity cannot be provided probably due to ununiformly distributed metal on the surface of pores of the crystalline silicoaluminophosphate.
Accordingly, the present inventors have made further studies and, as a result, have found that a method which comprises dispersing a crystalline silicoaluminophosphate in an aqueous active metal compound solution, spray-drying the dispersion and then calcining the spray-dried product at a high temperature can increase the amount of the metal supported and can produce a metal-supported catalyst that, even after hydrothermal treatment at elevated temperatures, does not cause a significant lowering in crystallinity and can develop a high level of activity. This has led to the completion of the present invention.
Further, it has been found that the modification of the metal-supported crystalline silicoaluminophosphate particles, obtained by the above method, with aluminum phosphate can provide a catalyst having high low-temperature activity and, in particular, supporting by the same spray drying method as described above, can provide a catalyst having high low-temperature activity. This has led to the completion of the present invention.